The present invention relates to a gas lift valve for use in an oil well producing by means of gas lift. More particularly, the present invention relates to a gas lift valve which makes use of a central body venturi for both controlling the flow of injection gas from an annulus between the tubing and the casing of the oil well, and precluding a reverse flow of fluids from the oil well to said annulus to occur.
Oil is usually found in accumulations under pressure in the subsoil, in porous and permeable sandstones known as reservoir stones, or simply reservoir, or yet producing rocking formations. Wells are drilled from the surface to drain off such reservoirs so as to communicate the reservoir with processing facilities in the surface, which are assembled to collect and to process the produced fluids.
Wells are bores which cross several rocking formations. Usually a steel pipe is inserted in such bores, named casing. At least one pipe of smaller diameter, named tubing, is inserted in such casing, through which fluids from the reservoir flow.
Oil is a complex mixture of heavy and light hydrocarbon phases, which may comprise from dry gas (methane) to heavy oil. Depending on the features of the reservoir, some components may appear in higher concentration than others. Some other substances may also accompany the produced oil, like water, carbon dioxide, hydrogen sulphide, salts and sand, etc.
Depending on the conditions of pressure and temperature, the constituents of the oil may be in a gaseous phase or in the liquid phase, or both. Thus, it should be concluded that the fluids that usually flow in an oil well may be considered as a multiphase multi component mixture.
The flow of fluids into an oil well, from the reservoir to the surface, occur as a consequence of the accumulated energy (pressure) in the reservoir, that is, without the presence of an external source of energy which provokes such production. In this case it is said that the well is flowing normally, or yet it is said that the well is producing by surge conditions. In case an external source of energy is used, e.g. a downhole pump, it is said that an artificial lift method is used.
Among the various known artificial lift methods, the continuous gas lift can be highlighted. In an usual configuration of this method, natural gas at high pressure is injected into an annulus formed between the casing and the tubing (or production string).
Valves known as gas lift valves are located at certain points of the tubing, which control the flow of gas flowing from the annulus to the interior of the tubing. The expansion of such pressurised gas and the consequent reduction of the multiphase mixture apparent specific gravity provide the necessary additional energy (pressure) to allow fluids from the reservoir to flow at a certain flow rate.
It is usual to control gas injection in an oil wells producing by continuous gas lift by means of a gas choke valve, located at the surface, and by another valve, which is the gas lift valve, located at the well bottom, at a certain location in the tubing.
Conventional gas lift valves used to control the rate of flow of injection gas in wells equipped to produce by means of continuous gas lift are not actually valves, although they are designated as valves by the experts and by the manufacturers. Actually they are flow regulators equipped with a small disc provided with a round orifice having a certain diameter. The edges of the orifice are usually sharp or smoothly rounded.
Such gas lift valves are also provided with a check valve, located downstream of the orifice, so as to preclude an undesirable flow of oil from the tubing to the annulus to occur.
Brazilian patent PI9300292-0, filed on Jan. 27, 1993 and commonly owned by the applicant of the present patent application, the description of which is herein incorporated for reference, disclosed an improved gas lift valve in which a venturi is used in place of the orifice of sharp edges usually used in conventional gas lift valves. According to this new conception, the irreversible losses of energy in the injection gas flow are significantly smaller, and a significant pressure recovery along the diffusor of the venturi occurs.
The critical flow of the injection gas is therefore achieved with a lower pressure head in the gas lift valve provided with a venturi than in a conventional gas lift valve, and thereby the flow rate of gas is kept constant more easily. As a consequence, the flow throughout the gas lift valve flows at a constant rate, whereby one of the worse operational problems occurring in oil well producing by means of continuous gas lift, the inconstancy of the flow rate, is overcome.
The ratio between the injection gas flow rate passing throughout the gas lift valve and the head of pressure between the intake port and the discharge port of the gas lift valve is usually referred to as the dynamic behaviour or dynamic performance of the gas lift valve. Thus, it can be said that a gas lift valve equipped with a venturi has a better dynamic performance than a gas lift valve equipped with an orifice.
Further, as a consequence of the lower pressure head required by the gas lift valve equipped with a venturi for injecting a certain rate of flow of gas, such gas lift valve provides a more rational use of energy, thereby provoking a reduction in the costs for compressing gas, considering the oil production flow rate being the same as the situation where a conventional gas lift valve is used, or instead augmenting the income by increasing the oil production flow rate, either by augmenting the injection gas flow rate or by injecting gas at a deeper location.
However, laboratory tests indicate that in many cases a good dynamic performance of the gas lift valve can be impaired by the check valve, which is usually located immediately after the venturi. Such check valve may cause a considerable constriction for the flow, in special in the situation where the features of the oil well require the use of venturis having throats of a large diameter for injecting significantly volumes of gas into the tubing.
The performance of a gas lift valve having a venturi decreases inasmuch as the diameter of the throat increases, due to the interference caused by the check valve, which, from a certain diameter of the throat on, exert a greater influence in the behaviour of the gas flow than the venturi.
The small space into a gas lift valve makes difficult to design a check valve which does not causes harmful effects to the dynamic performance of the gas lift valve. Moreover, as the check valve has movable parts in small spaces, such check valve is a jeopardy for a reliable operation of the gas lift valve, as a malfunctioning of the check valve can lead to an intervention in the oil well in order to replace the gas lift valve. In case the gas lift valve is installed in an undersea oil well, the costs for such intervention are very high.
The present invention proposes the use of a central body venturi which acts both as a venturi, enhancing the features of the injection gas flow, as previously mentioned, and also as a check valve, thereby eliminating the above drawbacks.
The present invention relates to a gas lift valve which makes use of a central body venturi for controlling the rate of the flow of injection gas and for preventing a reverse flow of fluids from the oil well to the annulus between the tubing and the casing of the oil well to occur.
The gas lift valve of the present invention should be used in a gas lift mandrel of an oil well producing by means of gas lift, the gas lift valve comprising:
a body;
a gas lift valve internal chamber;
at least one gas intake port for providing a passage for a flow of injection gas from an annulus between a casing and a tubing of said oil well to said gas lift valve internal chamber, said at least one gas intake port located in an upstream portion of said gas lift valve internal chamber; and
a hollow tip, connected to said gas lift valve internal chamber, said hollow tip provided with at least one gas discharge port;
said gas lift valve further comprising:
a central body venturi installed in said gas lift valve internal chamber, said central body venturi comprising:
a first upstream divergent segment, which provides, in said gas lift valve internal, chamber a progressive constriction in a cross sectional area for the passage of said flow of injection gas;
a second intermediate segment, located downstream of said first upstream segment, which provides into said gas lift valve internal chamber a substantially constant cross sectional area for the passage of said flow of injection gas, such area being substantially smaller than the original cross sectional area of said gas lift valve internal chamber;
a third convergent downstream segment, located downstream of said second intermediate segment, which provides into said gas lift valve internal chamber a progressive widening in the cross sectional area for the passage of said flow of injection gas until such cross sectional area becomes equal to the original cross sectional area of said gas lift valve internal chamber; and
a seat, located at said upstream portion of said gas lift valve internal chamber and downstream of said at least one gas intake port, said seat able to accommodate against its lower portion said first upstream segment of said central body venturi, thereby blocking off said gas lift valve and therefore precluding a reverse flow from said gas lift mandrel to said annulus to occur.
The central body venturi may be provided with primary and secondary fins for centring it in the gas lift valve internal chamber. Displacement limiters may also be provided for limiting the downward displacement of the central body venturi in the gas lift valve internal chamber.
A spring may be provided at the lower portion of the gas lift valve internal chamber for urging the central body venturi in a direction opposite to the direction of the flow of injection gas, so as to provide a faster blocking off of the gas lift valve in case a reverse flow occurs.